1. Field of the Invention
The present invention relates to bursting discs, and particularly although not exclusively to graphite bursting discs of the type used in the pharmaceutical preparation and chemical industries.
2. Description of Prior Art
It is well known in the chemical engineering and pharmaceutical industries to provide pressure relief devices for protecting pressure systems from over pressurization. One such pressure relief device is the known bursting disc, which comprises a substantially planar disc of graphite material which is installed at a suitable location in the pressurized system. When the pressure at one side of the disc rises above a predetermined design pressure, the graphite disc ruptures thereby releasing pressure from the system. One such typical application is on reaction vessels and chambers. In this case, the disc is attached to a flanged pipe outlet of a reaction chamber so that when the pressure in the reaction chamber rises above a predetermined design pressure, the graphite disc ruptures, releasing pressure in the reaction chamber.
Known bursting discs are of two types, the mono block type which is a singular item machined from a solid block of graphite and which has a peripheral outer ring 2 of graphite, and a thinner graphite membrane 4 extending across the ring, as shown in cut away bisected view in FIG. 1 of the accompanying drawings. Another type is the replacement element type bursting disc which comprises a circular graphite membrane in the form of a replaceable wafer disc, and a two part holder device into which the graphite membrane is arranged to fit. A cut away bisected view of a typical replacement type membrane is shown in FIG. 2 and comprises the membrane 6 and a ring gasket 8 on either side thereof.
Either type, is installed between pipework flanges. When the membrane ruptures or fractures, it relieves pressure from the reaction chamber or other pressurized installation.
In each type in order to create a differential pressure between one side of the bursting disc which faces the inside of the reaction chamber, and the other side of the bursting disc, which typically may lead to a venting pipe, the disc must create a gas impermeable barrier. As the graphite in its basic form is porous, to produce an effective bursting disc it is necessary to modify the graphite to make the membrane impermeable. If the graphite is porous, the response time of the disc to changes in differential pressure on opposite sides of the disc is poor, and the disc may fail to burst at all, or not burst quickly or at the correct pressure.
Conventionally, the graphite is impregnated with a thermo-setting resin, such as phenolics, PTFE polytetrafluoroethylene emulsions or a furanic resin.
Impregnation with such resins creates an impermeable bursting disc that gives a rapid bursting response as soon as the differential pressure between sides reaches a predetermined design pressure.
However, the resin impregnated graphite bursting disc is limited in operating temperature by the temperature at which the phenolic and furanic resins dissociate from the graphite. Typically such dissociation occurs at around 200.degree. C. and consequently, for constant operation of reactions at greater than 200.degree. C., known graphite bursting discs are unreliable. At such temperatures, small pores of resin dissociate from the graphite and the graphite becomes porous, leading to reduced bursting performance and contamination of the reaction chamber with resin debris.
Typically, known graphite bursting discs are used for reactions which occur at temperatures up to 165.degree. C.
It is also known to provide impregnated graphite bursting discs with a polytetrafluoroethylene (PTFE) coating or fluoropolymer liner to provide additional corrosion protection. However, such coated or lined impregnated discs are limited in operating temperature due to the impregnated graphite as described above and are only effective for use at temperatures up to 165.degree. C. Furthermore the coatings are porous and not impermeable to gas so that their use is only effective with impregnated or non-porous graphite.